Powder coating compositions containing a carboxylic acid functional polyester

ABSTRACT

Disclosed are improved curable powder coating compositions comprised of a particulate, film-forming mixture of a polymer having reactive functional groups, and a curing agent therefor having functional groups reactive with the functional groups of the polymer and present in an amount sufficient to cure the coating composition, wherein the inclusion of a particular polyester which has carboxylic acid functional groups improves adhesion to metal, particularly aluminum, and filiform corrosion resistance.

This is a Continuation-in-Part of copending application Ser. No.08/995,790, filed on Dec. 22, 1997, which is a Continuation-in-Part ofcopending application Serial No. 08/904,597, filed Aug. 1, 1997.

BACKGROUND OF THE INVENTION

The present invention relates to powder coating compositions withimproved adhesion and filiform corrosion resistance.

More specifically, the invention relates to a powder coating compositioncomprising a solid particulate film-forming mixture of a polymercontaining reactive functional groups and a curing agent therefor havingfunctional groups reactive with the functional groups of the polymerwith an additive to improve adhesion and filiform corrosion resistance.

Powder coating compositions for use in painting are extremely desirable.Such coating compositions can eliminate the organic solvents used inliquid paints. When the powder coating composition is thermally cured,little, if any, volatile material is given off to the surroundingenvironment. This is a significant advantage over liquid paints in whichorganic solvent is volatilized into the surrounding atmosphere when thepaint is cured by heating.

A particular problem which often results from the use of powdercoatings, particularly over aluminum substrates, is filiform corrosion.Filiform corrosion generally occurs in wet environments at the site of asurface defect in the presence of soluble ionic species. As described inFiliform Corrosion in Polymer-coated Metals, A. Bautista, PROGRESS INORGANIC COATINGS 28 at pages 49-58 (1996), this deterioration processgives rise to corrosion products which are characterized by afilamentous, worm-like appearance under the coatings. The "filaments"typically exhibit an arborescent structure and grow directionally underthe coating.

Filiform corrosion can result in delamination of the coating from ametal substrate and it has become a matter for increasing concern in theareas of automotive, industrial and architectural coatings. Accordingly,it is desirable to provide a powder coating composition with improvedfiliform corrosion. It has been found that incorporation of certainpolyesters having multiple carboxylic acid functional groups improvesthe filiform corrosion resistance of the powder coating composition.

Pending U.S. patent application Ser. No. 08/995,790 filed Dec. 22, 1997discloses a powder coating composition having improved filiformcorrosion resistance, wherein the improvement is due to the inclusion inthe composition of an organic polysiloxane having various pendantreactive functional groups. Examples of these pendant functional groupsinclude COOH, NCO, carbamate, primary and secondary amine and epoxyfunctional groups.

U.S. Pat. No. 5,543,464 teaches a powder coating composition comprisedof an epoxy functional group containing acrylic polymer, and, as curingagents, a crystalline polycarboxylic acid and a semi-crystallinepolycarboxylic acid group containing polyester. The polycarboxylic acidgroup containing polyesters are based on a polycondensation reaction of(cyclo) aliphatic and/or aromatic polyols with (cyclo) aliphatic and/oraromatic polycarboxylic acids or anhydrides, esters or acid chloridesbased on these acids. These acid functional polyester curing agentsimpart enhanced flexibility and improved impact resistance to theresultant coating.

U.S. Pat. No. 4,937,288 discloses a thermosetting powder coatingcomposition which comprises a co-reactable particulate mixture of acarboxylic acid group-containing acrylic polymer, a second carboxylicacid group-containing material with crystallinity sufficient to assistthe flow of the composition as it cures, and a beta-hydroxyalkylamidecuring agent. The second carboxylic acid group-containing material isselected from the class of C₄ to C₂₀ aliphatic dicarboxylic acids,polymeric polyanhydrides, and preferably, low molecular weightcarboxylic acid group-containing polyesters. The second carboxylic acidgroup-containing material provides flexibility and impact resistance ofthe resultant coating, as well as assisting in flow during cure of thepowder coating composition.

U.S. Pat. No. 5,407,707 discloses a powder coating composition whichcomprises a solid, particulate mixture of 60 to 90 percent by weight ofan epoxy functional copolymer and 10 to 40 weight percent of apolycarboxylic acid crosslinking agent. Suitable polycarboxylic acidcrosslinking agents include crystalline aliphatic materials such asadipic, succinic, sebacic, azelaic and dodecanedioic acid. Low molecularweight polycarboxylic acid group-containing polyesters and half-acidesters can also be used.

SUMMARY OF THE INVENTION

The curable powder coating composition of the present inventioncomprises a particulate film-forming mixture of a polymer containingreactive functional groups and a curing agent therefor having functionalgroups reactive with the functional groups of the polymer and beingpresent in an amount sufficient to cure the composition, the improvementcomprising the inclusion in the composition of a polyester havingmultiple carboxylic acid functional groups, said polyester comprising anoligomer having the following structure: ##STR1## wherein n is 2 to 4,and

R is C₇ to C₁₀ alkylene or a six-membered cyclic alkylene group,

and where the polyester is present in an amount sufficient to improvethe adhesion to metal and filiform corrosion resistance of the powdercoating composition, but insufficient to cure the coating composition inthe absence of the curing agent.

The polyester is the reaction product of pentaerithrytol and an excessof a dicarboxylic acid which has the following general structure:

    HOOC--R--COOH                                              (II)

wherein R is C₁₀ alkylene or a six-membered cyclic alkylene group.

The coating compositions of the invention are particularly useful forcoating aluminum substrates.

DETAILED DESCRIPTION OF THE INVENTION

As aforementioned, the polyester can be prepared by a polycondensationreaction of pentaerithrytol and an excess of a dicarboxylic acid havingthe formula (II), wherein R is C₇ to C ₁₀ alkylene or a six-memberedcyclic alkylene group.

Preferably, when R is C ₁₀ alkylene, the polyester is the reactionproduct of pentaerithrytol and dodecanedioic acid. When R is asix-membered cyclic alkylene group, the polyester is preferably thereaction product of pentaerithrytol and 1,4-cyclohexane dicarboxylicacid.

The pentaerithrytol and the dicarboxylic acid are reacted together at amolar ratio of from 1:6 to 1:8, preferably at a ratio of 1:8. A largeexcess of acid is desired to ensure an adduct with acid functionalityhaving a number average molecular weight (M_(n), as determined by gelpermeation chromatography using a polystyrene standard) in the range offrom 1000 to 4000, preferably from 2000 to 3000 The resultant lowmolecular weight polyester has an acid number of 310 to 420 (i.e., anacid equivalent weight of 130 to 180) and is a semi-crystalline solid atroom temperature. Further, the resultant polyester having carboxylicacid functionality has a melting range of from 115° C. to 130° C.,preferably from 118 ° C. to 123° C. By "semi-crystalline" is meantexhibiting a heterogeneous morphology, i.e., crystalline and amorphousphases; and typically opaque at ambient temperatures.

It should be appreciated that the above-described polycondensationreaction results in a complex mixture of monomers, i.e., unreactedpentaerithrytol and dicarboxylic acid; polyester oligomer and polyesterpolymer, which contains the polyester oligomer depicted in structure (I)above.

Besides the dicarboxylic acid, lower alkyl esters of the dicarboxylicacids can be used, such as C₁ ro 4 alkyl esters. Also, anhydride of thedicarboxylic acids, where they exist, can be used. Therefore, the termdicarboxylic acid is meant to include the acid itself and its functionalequivalents, the lower alkyl esters and the anhydrides.

The polyester having carboxylic acid functional groups is typicallypresent in the powder coating compositions in an amount sufficient toimprove the adhesion to metal and filiform corrosion resistance of thepowder coating composition, but insufficient to cure the coatingcomposition in the absence of a curing agent. Typically, the polyesteris present in an amount ranging from 1 to 30 weight percent, preferablyfrom 2 to 10 weight percent, and more preferably from 2 to 5 weightpercent, based on the total weight of resin solids in the powder coatingcomposition. It is, of course, understood that the polyester isdifferent from the polymers containing reactive functional groups andcuring agents which are described below.

Polymers Containing Reactive Functional Groups

The polymer containing reactive functional groups suitable for use inthe powder coating compositions of the invention can be chosen from avariety of materials, including, for example, acrylic polymers,polyurethane polymers, or polyester polymers having carboxylic acidfunctional groups, or acrylic polymers having epoxy functional groups.

In a preferred embodiment of the invention, the polymer containingreactive functional groups is a carboxylic acid group-containing acrylicpolymer. Such polymers can be formed by reacting a polymerizable alpha,beta-ethylenically unsaturated carboxylic acid with one or morepolymerizable, unsaturated monomers. Examples of carboxylic acidgroup-containing monomers which can be used are acrylic acid andmethacrylic acid, which are preferred, as well as crotonic acid,itaconic acid, fumaric acid, maleic acid, citraconic acid and the like,and monoalkylesters of unsaturated dicarboxylic acids.

Examples of other suitable monomers include vinyl aromatic compoundssuch as styrene, alkyl-substituted styrenes such as alpha-methylstyreneand halide-substituted styrene such as chlorostyrene. Other suitablepolymerizable, ethylenically unsaturated monomers which can be used areesters of acrylic and methacrylic acid such as methacrylate, ethylacrylate, butyl acrylate, 2-ethylhexyl acrylate, methyl methacrylate,ethyl methacrylate, N-butyl methacrylate, and 2-ethylhexyl methacrylate.In addition to the aforementioned ethylenically unsaturated,copolymerizable monomers, nitriles, such as acrylonitrile, vinyl andvinylidene halides such as vinyl chloride and vinylidene fluoride, andvinyl esters such as vinyl acetate may be used.

The carboxylic acid group containing acrylic polymers are typicallyprepared under conventional free radical initiated polymerizationconditions such as those described in U.S. Pat. No. 4,988,767 at column3, line 58 to column 4, line 8, incorporated herein by reference. Thecarboxylic acid group-containing acrylic polymers can also be preparedby other techniques well known in the art such as emulsionpolymerization, suspension polymerization, bulk polymerization orsuitable combinations thereof.

The polymer containing reactive functional groups can also be acarboxylic acid group-containing polyurethane polymer. Such polymers canbe prepared by reacting polyols and polyisocyanates so as to form apolyurethane polyol which is then reacted with polycarboxylic acid oranhydride to introduce free carboxyl groups into the reaction product.Examples of suitable polyols include aliphatic polyols such as ethyleneglycol, propylene glycol, butylene glycol, 1,6-hexylene glycol,neopentyl glycol, cyclohexanedimethanol, trimethylolpropane and thelike. Examples of suitable polyisocyanates are aromatic and aliphaticpolyisocyanates with the aliphatic polyisocyanates being preferred forexterior durability. Specific examples include 1,6-hexamethylenediisocyanate, isophorone diisocyanate and 4,4'-methylene-bis-(cyclohexylisocyanate). Examples of suitable polycarboxylic acids and anhydridesinclude succinic acid, adipic acid, azelaic acid, sebacic acid,terephthalic acid, isophthalic acid, tetrahydrophthalic acid,hexahydrophthalic acid, trimellitic acid and anhydrides of such acids.

In another preferred embodiment of the invention, the polymer containingreactive groups is a carboxylic acid group-containing polyester polymer.Such polyester polymers which are useful are those based on acondensation reaction of aliphatic polyols, including cycloaliphaticpolyols, with aliphatic and/or aromatic polycarboxylic acids andanhydrides. Examples of suitable aliphatic polyols include1,2-ethanediol, 1,3-propanediol, 1,4-butanediol, 1,6-hexanediol,neopentyl glycol, cyclohexane dimethanol, trimethylol propane, and thelike. Suitable polycarboxylic acids and anhydrides include those as aredescribed above in connection with the preparation of the carboxylicacid group-containing polyurethane. The polyol and the acid or anhydrideare reacted together with an excess of acid over alcohol so as to form apolyester which has free carboxylic acid groups.

In another preferred embodiment, the polymer containing reactivefunctional groups is an epoxy functional group-containing acrylicpolymer. Such epoxy functional group-containing acrylic polymers aretypically copolymers of an ethylenically unsaturated monomer having atleast one epoxy group and at least one polymerizable, ethylenicallyunsaturated monomer which is free of epoxy groups.

Examples of ethylenically unsaturated monomers containing epoxy groupsare those containing 1,2-epoxy groups and include glycidyl acrylate,glycidyl methacrylate and allyl glycidyl ether.

Examples of ethylenically unsaturated monomers which do not containepoxy groups are alkyl esters of acrylic and methacrylic acid containingform 1 to 20 carbon atoms in the alkyl group. Specific examples includemethyl methacrylate, ethyl methacrylate, butyl methacrylate, ethylacrylate, butyl acrylate, and 2-ethylhexyl acrylate.

Suitable other copolymerizable ethylenically unsaturated monomersinclude vinyl aromatic compounds such as styrene, and vinyl toluene;nitriles such as acrylonitrile and methacrylonitrile; vinyl andvinylidene halides such as vinyl chloride and vinylidene fluoride andvinyl esters such as vinyl acetate. Acid group-containingcopolymerizable ethylenically unsaturated monomers such as acrylic acidand methacrylic acid are preferably not used because of the possiblereactivity of the epoxy and acid groups.

The epoxy functional group-containing acrylic polymer is typicallyprepared by conventional means which are well known in the art, such asthose described in U.S. Pat. No. 4,703,101 at column 2, lines 29 to 44,incorporated herein by reference.

The various materials used to form the functional group containingpolymers of the present invention are selected such that the resultantmaterial has a high glass transition temperature (T_(g)), that is,greater than 30° C. The T_(g), which is a measure of the hardness andmelt flow of a polymer, can be calculated as described by Fox in Bull.Amer. Physics. Soc., 1,3 page 123 (1956). The T_(g) can also be measuredexperimentally and differential scanning calorimetry can be used (rateof heating 10° C. per minute, T_(g) taken at the first inflectionpoint). Unless otherwise indicated, the stated T_(g) as used hereinrefers to the calculated T_(g) in the case of acrylic polymers, andmeasured T_(g) in the case of condensation polymers such as polyestersand polyurethanes.

Typically, the polymer containing reactive functional groups is presentin the powder coating composition of the invention in an amount rangingfrom 20 to 97 weight percent, preferably from 50 to 85 weight percent,said weight percentages based on the total weight of resin solids in thepowder coating composition.

Curing Agents

As described above, the powder coating composition of the invention alsocomprises a curing agent having functional groups reactive with thefunctional groups of the polymer.

Polyepoxides as curing agents for carboxylic acid group-containingpolymers are well known in the art. Examples of polyepoxides suitablefor use as curing agents in the powder coating compositions of thepresent invention are those described in U.S. Pat. No. 4,681,811 atcolumn 5, lines 33 to 58, incorporated herein by reference.

Beta-hydroxyalkylamides as curing agents for carboxylic acidgroup-containing polymers are well known in the art. Examples ofbeta-hydroxyalkylamides suitable for use as curing agents in the powdercoating compositions of the invention are those described in U.S. Pat.No. 4,801,680 at column 2, line 42 to column 3, line 9, incorporatedherein by reference.

Also useful as a curing agent for carboxylic acid group-containingpolymers, as is well known in the art, is triglycidylisocyanurate(TGIC), a weatherable epoxy crosslinker commercially available asARALDITE TM PT-810 from Ciba-Geigy.

Polyacids, particularly polycarboxylic acids, as curing agents for epoxyfunctional group-containing acrylic polymers are well known in the art.Examples of polycarboxylic acids and polycarboxylic acidgroup-containing polyesters suitable for use as curing agents in thepowder coating compositions of the invention are those described in U.S.Pat. No. 5,407,707 at column 3, line 55 to column 4, line 10,incorporated herein by reference.

It is essential that the curing agent having functional groups reactivewith the functional groups of the polymer is present in an amountsufficient to cure the powder coating composition. Typically, the curingagent is present in the powder coating composition of the invention inan amount ranging from 2 to 50 weight percent, preferably from 5 to 20weight percent, said weight percentages based on the total weight ofresin solids in the powder coating composition.

The powder coating compositions of the present invention can optionallyinclude other materials such as pigments, fillers, light stabilizers,anti-oxidants and flow control agents and anti-popping agents.

A pigment can be included in the coating in amounts of up to 60 per centby weight based on total weight of the composition in order to give asuitable color to the resultant coating. Suitable pigments include, forexample, titanium dioxide, ultramarine blue, phthalocyanine blue,phthalocyanine green, carbon black, graphite fibrils, black iron oxide,chromium green oxide, ferride yellow and quindo red.

In addition, the powder coating composition may include fumed silica orthe like to reduce caking of the powder during storage. An example of afumed silica is commercially available from Cabot Corporation under thetrademark CAB-O-SIL. The fumed silica is present in amounts ranging upto 1 percent by weight based on total weight of the powder coatingformulations.

For good exterior durability, the compositions also can containultraviolet light absorbing agents, ultraviolet light stabilizers andantioxidants. Such materials are commercially available from Ciba-Geigyunder the trademarks TINUVIN and IRGANOX. The ultraviolet lightabsorbing agents, ultraviolet light stabilizers and antioxidants, whenused, are typically present in the compositions individually in amountsup to 6 percent by weight based on weight of resin solids.

One group of suitable flow control agents are acrylic polymers such aspolylauryl acrylate, polybutyl acrylate, poly(2-ethylhexyl) acrylate,poly(ethyl-2-ethylhexyl) acrylate, polylauryl methacrylate andpolyisodecenyl methacrylate. The flow control agent may also be afluorinated polymer such as an ester of polyethylene glycol orpolypropylene glycol and fluorinated fatty acids, for example, an esterof polyethylene glycol of a molecular weight of over 2,500 andperfluorooctanoic acid. Polymeric siloxanes of molecular weights over1,000 may also be used as a flow control agent, for example,poly(dimethylsiloxane) or poly(methylphenyl)siloxane. The flow controlagent, when used, is present in amounts up to 5 percent by weight basedon total weight of the coating composition.

Anti-popping agents can be added to the composition to allow anyvolatile material to escape from the film during baking. Benzoin is apreferred anti-popping agent and when used is generally present inamounts up to 3.0 percent by weight based on total weight of the powdercoating composition.

The powder coating compositions are typically prepared by melt blendingthe ingredients. This can be accomplished by first blending theingredients in a high shear mixer such as a planetary mixer, and thenmelt blending in an extruder from 80° C. to 130° C. The extrudate isthen cooled and pulverized into a particulate material which can beapplied by spraying.

The particulate powder coating compositions can be applied directly to asubstrate of, for example, metal such as steel or aluminum, or to aprimed metal substrate. In particular, when the particulate coatingcompositions of the invention are applied to unprimed aluminumsubstrates as clear coats, an improvement in adhesion and filiformcorrosion resistance is noted. Application can be by spraying, and inthe case of a metal substrate, by electrostatic spraying which ispreferred, or by the use of a fluidized bed. The coating composition canbe applied as a primer or as a primer surfacer, or as a topcoat or as afinishing coat. The powder coating can be applied in a single sweep orin several passes to provide a film having a thickness after cure offrom 1 to 10 mils (25.4 to 254 microns), usually 2.0 to 4.0 mils (50.8to 100.4 microns).

After application of the powder coating composition, the powder coatedsubstrate is baked at a temperature sufficient to cure the product,typically at 250° F. to 400° F. (121° to 204° C.) for 1 to 60 minutes,and preferably at 300° F. to 350° F. (160° to 175° C.) for 15 to 30minutes.

The following examples illustrate the invention and should not beconstrued as a limitation on the scope thereof. Unless specificallyindicated otherwise, all percentages and amounts are by weight.

EXAMPLES

Examples 1 through 5 describe the preparation of various carboxylic acidfunctional polyesters. Example 1 describes the preparation of acarboxylic acid functional polyester based on pentaerithrytol anddodecanedioic acid. Example 2 describes the preparation of a carboxylicacid functional polyester based on pentaenthrytol and 1,4-cyclohexanedicarboxylic acid and Example 3 describes the preparation of acarboxylic acid functional polyester based on 1,6-hexanediol anddodecanedioic acid. Examples 5 and 6 describe the preparation ofpolyesters based on pentaerithrytol and adipic acid, and di-trimethylolpropane and adipic acid, respectively.

Examples A through D describe the preparation of various powdercompositions. Example A describes the preparation of two powder coatingcompositions based on a carboxylic acid functional acrylic polymer and apolyepoxide curing agent. Example A-1 describes a powder coatingcomposition containing the carboxylic acid functional polyester ofExample 1 for improved filiform corrosion resistance, while ComparativeExample A-2 contains no polyester.

Example B describes the preparation of six powder coating compositionsbased on an epoxy functional group containing acrylic polymer and acarboxylic acid group containing curing agent. Each of the Examples B-1through B-5 contains a carboxylic functional polyester of Example 1through Example 5, respectively. Comparative Example B-6 contains nopolyester.

Example C describes the preparation of four powder coating compositionsbased on a carboxylic acid functional polyester polymer and abeta-hydroxyalkylamide curing agent. Each of Examples C-1 through C-3incorporates a polyester of Example 1 through Example 3, respectively.Comparative Example C-4 contains not carboxylic acid functionalpolyester.

Example D describes the preparation of two powder coating compositionsbased on an epoxy functional group containing acrylic polymer and acarboxylic acid functional materials. Example D-1 contains both thecarboxylic acid functional polyester of Example 1 and a carboxylic acidfunctional curing agent, while Comparative Example D-2 contains only thecarboxylic acid functional polyester with no curing agent.

Example 1

This example describes the preparation of a carboxylic acid functionalpolyester based on pentaerithrytol and dodecanedioic acid. The polyesteris prepared from a mixture of the following ingredients:

    ______________________________________                                                    Parts by Weight        Equivalent                                   Ingredients: (grams) % on Solids Weight                                     ______________________________________                                        Pentaerithrytol                                                                           544.64       6.88%      34.04                                       Dodecanedioic acid 7360.00 92.92%  115.00                                     Dibutyl tin oxide 7.90 0.10% --                                               Triphenyl phosphite 7.90 0.10% --                                             Total 7920.45 100%                                                          ______________________________________                                    

The above ingredients were combined in a suitable reaction vesselequipped with a thermocouple, stirrer, nitrogen gas inlet and a partialcondenser with a distilling head. Under a light nitrogen sparge, thereaction mixture was heated to a temperature of 151° C. over a period of3.5 hours, whereupon water began to distill off. Heating was continuedas the temperature increased to 180° C. over a one hour period. Thattemperature was held over a period of 3.5 hours to a stalled acid valueof about 352. At this time approximately 218.0 grams of water had beencollected.

The polyester thus prepared was a fluid liquid which upon discharge fromthe reaction flask, was permitted to cool, yielding a semi-crystallinesolid with an acid value of 351.5.

Example 2

This example describes the preparation of a carboxylic acid functionalpolyester based on pentaerithrytol and 1,4-cyclohexane dicarboxylicacid. The polyester is prepared from a mixture of the followingingredients:

    ______________________________________                                                    Parts by Weight        Equivalent                                   Ingredients: (grams) % on Solids Weight                                     ______________________________________                                        Pentaerithrytol                                                                           204.24       8.98%     34.04                                        Cyclohexanediacid 2066.16 90.82%   86.09                                      Dibutyl tin oxide 2.27 0.10% --                                               Triphenyl phosphite 2.27 0.10% --                                             Total 2274.94 100%                                                          ______________________________________                                    

The above ingredients were combined in a suitable reaction vesselequipped with a thermocouple, stirrer, nitrogen gas inlet and a partialcondenser with a distilling head. Under a light nitrogen sparge, thereaction mixture was heated to a temperature of about 158° C. over aperiod of about 2.5 hours, whereupon water began to distill off. Heatingwas continued as the temperature increased to about 200° C. over a 0.75hour period. That temperature was held over a period of about 0.5 hourto a stalled acid value of about 466. At this time approximately 83.0grams of water had been collected.

The polyester thus prepared was a fluid liquid which upon discharge fromthe reaction flask, was permitted to cool, yielding a semi-crystallinesolid with an acid value of 462.9.

Example 3

This example describes the preparation of a carboxylic acid functionalpolyester based on 1,6-hexanediol and dodecanedioic acid. The polyesteris prepared from a mixture of the following ingredients:

    ______________________________________                                                    Parts by Weight        Equivalent                                   Ingredients: (grams) % on Solids Weight                                     ______________________________________                                        1,6-Hexanediol                                                                            826.00       29.28%     59.00                                       Dodecanedioic acid 1993.18 70.67% 115.00                                      Dibutyl tin oxide 1.41  0.05% --                                              Total 2820.59 100%                                                          ______________________________________                                    

The above ingredients were combined in a suitable reaction vesselequipped with a thermocouple, stirrer, nitrogen gas inlet and a partialcondenser with a distilling head. Under a light nitrogen sparge, thereaction mixture was heated to a temperature of about 151° C. over aperiod of about 2 hours, whereupon water began to distill off. Heatingwas continued as the temperature increased to about 220° C. over about a3.5 hour period. That temperature was held over a period of about 1.5hours to a stalled acid value of about 72.8. At this time approximately215 grams of water had been collected.

The polyester thus prepared was a fluid liquid which upon discharge fromthe reaction flask, was permitted to cool, yielding a semi-crystallinesolid with an acid value of 76.2.

Example 4

This example describes the preparation of a carboxylic acid functionalpolyester based on pentaerithrytol and adipic acid. The polyester isprepared from a mixture of the following ingredients:

    ______________________________________                                                    Parts by Weight        Equivalent                                   Ingredients: (grams) % on Solids Weight                                     ______________________________________                                        Pentaerithrytol                                                                           204.24       10.42%    34.04                                        Adipic acid 1752.00 89.38% 73.00                                              Dibutyl tin oxide 1.96  0.10% --                                              Triphenyl phosphite 1.96  0.10% --                                            Total 1960.15 100%                                                          ______________________________________                                    

The above ingredients were combined in a suitable reaction vesselequipped with a thermocouple, stirrer, nitrogen gas inlet and a partialcondenser with a distilling head. Under a light nitrogen sparge, thereaction mixture was heated to a temperature of about 156° C. over aperiod of about 2.0 hours, whereupon water began to distill off. Heatingwas continued as the temperature increased to about 200° C. over a 1.0hour period. That temperature was held over a period of about 0.5 hourto a stalled acid value of about 545.2. At this time approximately 83.0grams of water had been collected.

The polyester thus prepared was a fluid liquid which upon discharge fromthe reaction flask, was permitted to cool, yielding a semi-crystallinesolid with an acid value of 537.5.

Example 5

This example describes the preparation of a carboxylic acid functionalpolyester for use as an additive in the powder coating compositions ofthe invention. The polyester is prepared from a mixture of the followingingredients:

    ______________________________________                                                    Parts by Weight        Equivalent                                   Ingredients: (grams) % on Solids Weight                                     ______________________________________                                        Di-trimethylol propane                                                                    190.17       17.80%    63.39                                        Adipic acid 876.00 82.00% 73.00                                               Dibutyl tin oxide 1.07  0.10% --                                              Triphenyl phosphite 1.07  0.10% --                                            Total 1068.30 100%                                                          ______________________________________                                    

The above ingredients were combined in a suitable reaction vesselequipped with a thermocouple, stirrer, nitrogen gas inlet and a partialcondenser with a distilling head. Under a light nitrogen sparge, thereaction mixture was heated to a temperature of about 157 C. over aperiod of about 1.5 hours, whereupon water began to distill off. Heatingwas continued as the temperature increased to about 180° C. over a 1.25hour period. That temperature was held over a one hour period to astalled acid value of about 497.8. At this time approximately 34.0 gramsof water had been collected.

The polyester thus prepared was a fluid liquid which upon discharge fromthe reaction flask, was permitted to cool, yielding a semi-crystallinesolid with an acid value of 497.3.

Powder Coating Composition Testing Procedures

Each of the following powder coating compositions was electrostaticallyapplied to cleaned only aluminum substrate (commercially available fromACT, Inc. as A407A1), then cured as described below. The powder coatedpanels were then tested for various physical properties to includesolvent resistance/extent of cure, 20° gloss, filiform corrosionresistance and general appearance. The powder coating formulations weretested for stability and thermal shock resistance.

Solvent resistance/extent of cure was tested according to ASTM D5402using methyl ethyl ketone (MEK) double rubs. Results are reported forappearance and mar after 200 double rubs, or, alternately, as the numberof double rubs completed before breaking through the coating to thesubstrate. 20° gloss was determined using a BYK-Gardner haze-glossmeter.Appearance was evaluated via visual inspection.

Filiform corrosion resistance was tested by scribing the cured coatedsubstrate, exposing the scribed test panel in the Copper AcceleratedAcetic Acid Salt Spray ("CASS") test cabinet according to ASTM B368-68for 6 hours, then thoroughly rinsing the panel with deionized water.These rinsed panels were subsequently exposed to an 85% relativehumidity/60° C. environment for a period of up to 4 weeks. Resultsreported represent the average length (in millimeters) of corrosionfiliments as measured outward from the scribe line.

Powder stability was tested by placing a sealed 2 ounce sample of thepowder coating composition in a water bath at 40° C. for one week. Thepowder was then examined for caking and/or fusing together of powderparticles. Thermal shock resistance was tested by soaking cured powdercoated panels in water at 100° F. for 4 hours, then immediatelytransferring the panels to a 30° C bath to cool. Once cooled, panelswere scribed and within 30 seconds the scribed area was exposed to a 5psi steam blast. Panels were then visually examined for blushing, waterspotting and adhesion loss. Results are reported as pass/fail.

Example A

This example describes the preparation of two powder coatingcompositions based on a carboxylic acid functional acrylic polymer and apolyepoxide curing agent. Example A-1 contains the carboxylic acidfunctional polyester for improved filiform corrosion resistance, andComparative Example A-2 contains no polyester. Each of the examples wasprepared from a mixture of the following ingredients:

    ______________________________________                                                      Example A-1                                                                             Comparative Example A-2                                 Ingredients: (grams) (grams)                                                ______________________________________                                        TGIC.sup.1    67.8      41.6                                                    CRYLCOAT 450.sup.2 357.0 432.5                                                Pentasiloxane.sup.3 30.4 30.3                                                 Polyester of Example 1 50.8 --                                                URAFLOW B.sup.4 5.2 5.2                                                       IRGANOX 1076.sup.5 5.1 5.1                                                    RESIFLOW PL-200.sup.6 5.6 5.6                                                 TROY 570.sup.7 5.2 5.2                                                      ______________________________________                                         .sup.1 Triglycidylisocyanurate, commercially available from CYTEC Corp.       .sup.2 Acid functional polyester, commercially available from UCB             Chemicals..                                                                   .sup.3 Acid functional polysiloxane (SiO).sub.5, the preparation of which     is described in Example 2 of U.S. patent application Ser. No. 08/995,790,     filed 22 December 1977.                                                       .sup.4 Benzoin, commercially available from Monsanto Chemical Co.             .sup.5 Polyphenol antioxidant available from CibaGeigy Corp.                  .sup.6 Silica/acrylic polymer dispersion, a flow control additive             available from Estron Chemical, Inc.                                          .sup.7 Silicone/amide flow control additive, available from Troy Chemical     Corp.                                                                    

The ingredients of each of the Examples A-1 and A-2 immediately abovewere mixed via typical powder compounding techniques. Each powdercomposition was electrostatically applied to cleaned only aluminumsubstrate then cured at 340° F. (171° C.) for 20 minutes, and tested asdescribed above for filiform corrosion resistance, haze rating, hazecracking and 20° gloss. The following Table 1 illustrates the advantagesfor improved filiform corrosion resistance, while maintaining otherperformance properties, obtained by the incorporation of the carboxylicacid functional polyester into the powder coating composition.

                  TABLE 1                                                         ______________________________________                                                                  Example A-2                                           Test performed: Example A-1 (Comparative)                                   ______________________________________                                        20° Gloss                                                                          89            88                                                    Thermal shock Pass Fail                                                       Filiform corrosion 5 mm, medium density 8 mm, high density                  ______________________________________                                    

Example B

This example describes the preparation of six powder coatingcompositions based on an epoxy functional group containing acrylicpolymer and a carboxylic acid group containing curing agent. Each of theExamples B-1 through B-5 contains a carboxylic acid functional polyesterof Example 1 through Example 5, respectively. Comparative Example B-6contains no polyester. Each of the examples was prepared from a mixtureof the following ingredients:

    ______________________________________                                                Exam-   Exam-   Exam- Exam- Exam- Example                                ple ple ple ple ple B-6                                                       B-1 B-2 B-3 B-4 B-5 com-                                                     Ingredients: (grams) (grams) (grams) (grams) (grams) parative               ______________________________________                                        Polyester of                                                                          65.6    --      --    --    --    --                                    Example 1                                                                     Polyester of --  65.6 -- -- -- --                                             Example 2                                                                     Polyester of -- --  65.6 -- -- --                                             Example 3                                                                     Polyester of -- -- --  65.6 -- --                                             Example 4                                                                     Polyester of -- -- -- --  65.6 --                                             Example 5                                                                     Dode- 95.7 108.6 63.8 117.2 112.5 65.6                                        canedioic                                                                     acid                                                                          GMA acrylic 384.8 371.8 416.7 363.4 367.9 414.8                               resin.sup.1                                                                   EPON 23.0 23.0 23.0 23.0 23.0 23.0                                            1001F.sup.2                                                                   URAFLOW 2.9 2.9 2.9 2.9 2.9 2.9                                               B                                                                             TINUVIN 2.4 2.4 2.4 2.4 2.4 2.4                                               900.sup.3                                                                     TROY 570 2.4 2.4 2.4 2.4 2.4 2.4                                            ______________________________________                                         .sup.1 ALMATEX A207S available from Reichold Chemicals, Inc.                  .sup.2 Polyglycidyl ether of Bisphenol A, having an equivalent weight of      1000, available from Shell Oil and Chemical Co.                               .sup.3 Micronized                                                             2(2hydroxy-benzotriazol-2-yl)-4,6-bis(methyl-1-phenylethyl)phenol, an         ultraviolet absorber light stabilizer available from CibaGeigy Corp.     

The ingredients of each of the Examples B-1 through B-6 immediatelyabove were mixed via typical powder compounding techniques. Each powdercomposition was electrostatically applied to cleaned only aluminumsubstrate then cured at 340° F. (171° C.) for 20 minutes, and tested asdescribed above for filiform corrosion resistance, haze rating,stability, thermal shock and 20° gloss. The following Table 2illustrates the advantages of improved filiform corrosion resistance,while maintaining other performance properties, obtained by theincorporation of the particular carboxylic acid functional polyesters ofExample 1 and Example 2 into the powder coating composition.

                  TABLE 2                                                         ______________________________________                                                Exam-   Exam-   Exam- Exam- Exam- Exam-                                 Test ple ple ple ple ple ple                                                  Performed: B-1 B-2 B-3 B-4 B-5 B-6                                          ______________________________________                                        Appearance                                                                            Clear;  Exu-    Clear;                                                                              Heavy Powder                                                                              Clear;                                 sharp date; sharp exudate too sharp                                            dull   un-                                                                       stable*                                                                  Stability 7 days, 7 days, 7 days, 7 days, -- 7 days,                          @35° C. good good slight cake  good                                       cake                                                                       Thermal pass, fail, pass fail, -- pass                                        shock slight chip  slight                                                      haze   chip,                                                                     haze                                                                      Filiform 2 mm., 5 mm., 5.5 0 mm., -- 9 mm.,                                   corrosion low low mm., heavy  high                                             density density low exudate  density                                            density                                                                  ______________________________________                                         *It should be noted that the carboxylic acid functional polyester of          Example 5 was very soft, hence the powder coating composition which           incorporated this material was unstable. As a result, no data was             generated for powder coating composition of Example B5.                  

Example C

This example describes the preparation of four powder coatingcompositions based on a carboxylic acid functional polyester polymer anda beta-hydroxyalkylamide curing agent. Each of the Examples C-1, C-2 andC-3 incorporates a carboxylic acid functional polyester (different fromthe aforementioned carboxylic acid functional polymer), and ComparativeExample C-4 contains no carboxylic acid functional polyester. The powdercoating compositions were prepared from a mixture of the followingingredients:

    ______________________________________                                                  Example  Example   Example                                                                              Example C-4                                  C-1 C-2 C-3 (comparative)                                                    Ingredients: (grams) (grams) (grams) (grams)                                ______________________________________                                        Polyester of                                                                            31.3     --        --     --                                          Example 1                                                                     Polyester of --  31.3 -- --                                                   Example 2                                                                     Polyester of -- -- 31.3 --                                                    Example 3                                                                     PRIMID AL-552.sup.1 40.9 45.8 28.7 26.9                                       DSM-P800.sup.2 453.1 448.1 465.0 498.1                                        URAFLOW B 2.0 2.0 2.0 2.0                                                     MICROWAX C.sup.3 4.5 4.5 4.5 4.5                                              TINUVIN 900 11.0 11.3 11.3 11.3                                               TINUVIN 144.sup.4 5.7 5.7 5.7 5.7                                             TROY 570 5.7 5.7 5.7 5.7                                                      HCA-1.sup.5 5.7 5.7 5.7 5.7                                                   Total 560 554 554 560                                                       ______________________________________                                         .sup.1 Betahydroxyalkylamide curing agent, available from EMSAmerican         Grilon, Inc.                                                                  .sup.2 Ultradurable acid functional polyester available from DSM Resins.      .sup.3 Wax C MicroPowder, a fatty acid amide (bisstearamide of ethylene       diamine) available from HoechstCelanese.                                      .sup.4                                                                        2tert-butyl-2-(4-hydroxy-3,5-di-tert-butylbenzyl)[bis(methyl2,26,6-tetram    thyl-4-piperinyl)dipropionate, an ultraviolet light stabilizer available       from CibaGeigy Corp.                                                          .sup.5 9,10dihydro-9-oxa-10-phosphaphenanthene-10-oxide (or                   3,4,5,6dibenzo-1,2-oxaphosphane-2-oxide), an antiyellowing agent availabl     from Sanko Chemical Co., Ltd.                                            

The ingredients of each of the Examples C-1 through C-4 immediatelyabove were mixed via typical powder compounding techniques. Each powdercomposition was electrostatically applied to cleaned only aluminumsubstrate then cured at 340° F. (171° C.) for 20 minutes, and tested asdescribed above for filiform corrosion resistance, haze rating, hazecracking, stability, thermal shock and 20° gloss. The following Table 3illustrates the improvement in filiform corrosion resistance, whilemaintaining other performance properties, obtained by the incorporationof the particular carboxylic acid functional polyesters of Example 1 andExample 2 into the powder coating composition.

                  TABLE 3                                                         ______________________________________                                        Test    Example   Example   Example Example C-4                                 Performed: C-1 C-2 C-3 (comparative)                                        ______________________________________                                        20° Gloss                                                                      129       126       124     134                                         Haze rating 458 457 402 472                                                   Haze Pass Pass Fail Pass                                                      cracking                                                                      Stability 7 days, 7 days, 7 days, 7 days,                                     @35° C. excellent excellent excellent excellent                        Thermal Pass Pass Fail, slight Fail, slight                                   shock   edge lift edge lift                                                   Filiform 1 mm., low 5 mm., low 8 mm., high 10 mm., high                       corrosion density density density density                                   ______________________________________                                    

Example D

This example describes the preparation of two powder coatingcompositions based on an epoxy functional group containing acrylicpolymer and a carboxylic acid functional curing agent. ExampleD-1contains both the carboxylic acid functional polyester of Example 1and a carboxylic acid functional curing agent, while Comparative ExampleD-2 contains only the carboxylic acid functional polyester with no acidfunctional curing agent. The powder coating compositions were preparedfrom a mixture of the following ingredients:

    ______________________________________                                                                   Comparative                                           Example D-1 Example D-2                                                      Ingredients: (grams) (grams)                                                ______________________________________                                        Polyester of Example 1                                                                         70.6      70.6                                                 Dodecanedioic acid 103.0 --                                                   GMA acrylic resin.sup.1 414.4 414.4                                           EPON 1001F 35.4 35.4                                                          URAFLOW B 4.4 4.4                                                             TINUVIN 900 3.7 3.7                                                           TROY 570 3.7 3.7                                                            ______________________________________                                         .sup.1 ALMATEX A207S available from Reichold Chemicals, Inc.             

The ingredients of each of the Examples D-1 and D-2 immediately abovewere mixed via typical powder compounding techniques. Each powdercomposition was electrostatically applied to cleaned only aluminumsubstrate then cured at 340° F. (171° C.) for 20 minutes, and tested asdescribed above for appearance (rated by visual inspection), hazerating, 20° gloss, and extent of cure. The following Table 4 illustratesthat the carboxylic acid functional polyester is present in the powdercoating compositions of the invention in an amount insufficient to curethe coating composition in the absence of the curing agent.

                  TABLE 4                                                         ______________________________________                                        Test performed:                                                                             Example D-1                                                                              Example D-2                                          ______________________________________                                        Appearance    smooth, clear                                                                            smooth, clear                                          MBK solvent rubs (200×) Pass, slight Fail at 100 rubs, complete                                  scratch break through                                20° Gloss 128 128                                                      Haze rating 457 456                                                         ______________________________________                                    

What is claimed is:
 1. In a curable powder coating compositioncomprising a particulate film-forming mixture of a polymer containingreactive functional groups and a curing agent therefor having functionalgroups reactive with the functional groups of the polymer and beingpresent in an amount sufficient to cure said polymer, the improvementcomprising the inclusion in said composition of a polyester havingcarboxylic acid functional groups, said polyester comprising an oligomerhaving the following structure: ##STR2## and where the polyester ispresent in an amount ranging from 2 to 5 weight percent based on totalweight of resin solids in the powder coating composition, said amountbeing sufficient to improve the adhesion to metal and filiform corrosionresistance of the powder coating composition, but insufficient to curethe coating composition in the absence of the curing agent.
 2. In acurable powder coating composition comprising a particulate film-formingmixture of a polymer containing reactive functional groups and a curingagent therefor having functional groups reactive with the functionalgroups of the polymer and being present in an amount sufficient to curesaid polymer, the improvement comprising the inclusion in saidcomposition of a polyester having carboxylic acid functional groups,said polyester being the reaction product of the following reactants:(a)pentaerithrytol; and (b) a dicarboxylic acid having the followingstructure:

    HOOC--R--COOH

wherein R is C₇ to C₁₀ alkylene or a six-membered cyclic alkylene group,and where the polyester is present in an amount ranging from 2 to 5weight percent based on total weight or resin solids in the powdercoating composition, said amount being sufficient to improve theadhesion to metal and filiform corrosion resistance of the powdercoating composition, but insufficient to cure the coating composition inthe absence of the curing agent.
 3. The powder coating composition ofclaim 2 wherein when R is C₁₀ alkylene.
 4. The powder coatingcomposition of claim 2 wherein when R is a six-membered cyclic alkylenegroup.
 5. The powder coating composition of claim 1 wherein the polymercontaining reactive functional groups is selected from the groupconsisting of a polyester polymer having carboxylic acid functionalgroups and an acrylic polymer having carboxylic acid functional groups.6. The powder coating composition of claim 5 wherein the curing agent isa beta-hydroxyalkylamide.
 7. The powder coating composition of claim 5wherein the curing agent is triglycidylisocyanurate.
 8. The powdercoating composition of claim 5 wherein the curing agent is apolyepoxide.
 9. The powder coating composition of claim 1 wherein thepolymer containing reactive functional groups is an acrylic polymerhaving epoxy functional groups.
 10. The powder coating composition ofclaim 9 wherein the curing agent is a polycarboxylic acid different fromsaid polyester.
 11. A coated article comprising an aluminum substrateand a cured coating thereon, said cured coating being derived from acurable powder coating composition comprising a particulate film-formingmixture of a polymer containing reactive functional groups and a curingagent therefor having functional groups reactive with the functionalgroups of the polymer and being present in an amount sufficient to curesaid polymer, wherein the improvement comprises the inclusion in saidcomposition of a polyester comprising an oligomer having the followingstructure: ##STR3## and where the polyester is present in an amountranging from 2 to 5 weight percent based on total weight of resin solidsin the powder coating composition, said amount being sufficient toimprove the adhesion to metal and filiform corrosion resistance of thepowder coating composition, but insufficient to cure the coatingcomposition in the absence of the curing agent.
 12. A coated articlecomprising an aluminum substrate and a cured coating thereon, said curedcoating being derived from a curable powder coating compositioncomprising a particulate film-forming mixture of a polymer containingreactive functional groups and a curing agent therefor having functionalgroups reactive with the functional groups of the polymer and beingpresent in an amount sufficient to cure said polymer, wherein theimprovement comprises the inclusion in said composition of a polyesterhaving carboxylic acid functional groups, said polyester being thereaction product of the following reactants:(a) pentaerithrytol; and (b)a dicarboxylic acid having the following structure:

    HOOC--R--COOH

wherein R is C₇ to C₁₀ alkylene or a six-membered cyclic alkylene group,and where the polyester is present in an amount ranging from 2 to 5weight percent based on total weight of resin solids in the powdercoating composition, said amount being sufficient to improve theadhesion and filiform corrosion resistance of the powder coatingcomposition, but insufficient to cure the coating composition in theabsence of the curing agent.
 13. The coated article of claim 12 whereinwhen R is C₁₀ alkylene.
 14. The coated article of claim 12 wherein whenR is a six-member cyclic alkylene group.
 15. The coated article of claim12 wherein the polymer containing reactive functional groups is selectedfrom the group consisting of a polyester polymer having carboxylic acidfunctional groups and an acrylic polymer having carboxylic acidfunctional groups.
 16. The coated article of claim 15 wherein the curingagent is a beta-hydroxyalkylamide.
 17. The coated article of claim 15wherein the curing agent is triglycidylisocyanurate.
 18. The coatedarticle of claim 15 wherein the curing agent is a polyepoxide.
 19. Thecoated article of claim 13 wherein the polymer containing reactivefunctional groups is an acrylic polymer having epoxy functional groups.20. The coated article of claim 19 wherein the curing agent is apolycarboxylic acid different from said polyester.